In the design of DC/DC board mounted power supplies, the maximum output current of the power supply generally has a direct effect on the size of the power supply. The available space, more particular the height, for the power supply is becoming increasingly limited as equipment manufacturers continue to respond to their customers' demands for more power in a smaller "packet." One significant limitation in the reduction of the DC/DC board mounted power supplies is the power transformer employed to step-up or, more commonly, step-down a DC input voltage to the required DC output voltage needed to power the equipment load. Reducing the size of the power transformer typically has a corresponding effect of reducing the maximum output current of the power supply.
One conventional approach to overcome the above-described limitations has been to replace the single power transformer with multiple smaller power transformers. Additionally, multiple power converters with smaller power transformers may be parallel-coupled to provide a single output; the maximum output current is a combination of the maximum output current of each individual power converter. This topology provides flexibility in the design of board mounted power supply in that the number of constituent converters can be raised or lowered depending on the equipment's current load requirement.
The use of parallel-coupled converters in a power supply, however, is not without concerns. One important consideration is the current balance between the parallel-coupled converters. For example, if a power supply has two parallel-coupled converters, typically each converter is designed to deliver half of the load current. In the event that one of the converters is not delivering half of the load current (e.g., a quarter of the load current), the other converter has to "make up" the difference. The problem arises when the converter is forced to deliver a current at or above its design output load current; the stress and heat from prolonged use may damage or, at the very least, shorten the operational life of the converter. Additionally, a converter that is forced to compensate for another converter may not be able to provide the required current (i.e., the current exceeds the output current capacity of the converter). In this event, the load equipment may not function properly, if at all.
A conventional approach to balancing the currents in the parallel-coupled converters is to employ current sensing resistors (a current shunt) in each converter, to sense the load current. The sensed load currents are then compared to one another to provide an "equalizing" signal that is used to adjust the operation of the converters to balance the output currents therefrom. To adequately balance the currents, the resistance values of the current shunt must be designed within very "tight" tolerances. To design the resistors within tight tolerances, however, is typically costly and results in a more expensive power supply design.
Accordingly, what is needed in the art is a control system for a power supply having parallel-coupled converters that overcomes the above-described limitations.